Vehicle posture control apparatus

ABSTRACT

In a posture control apparatus controlling the posture of a vehicle in yawing direction by independently controlling brakes of the wheels, intervention of a first oversteer control is carried out when the oversteering tendency of the vehicle is stronger than a first preset reference value, intervention of a second oversteer control, in which the control amount is lower than in the first oversteer control, is carried out when the oversteering tendency of the vehicle is not stronger than the first preset reference value, but stronger than a second preset reference value, and intervention of a third oversteer control, in which the control amount is lower than in the first oversteer control, is carried out when the oversteering tendency of the vehicle is not stronger than the second preset reference value, but stronger than a third preset reference value.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the technological field ofvehicle posture control apparatuses that avoid or suppress understeeringtendencies (drifting out) and oversteering tendencies (spinning) bycontrolling the posture of a vehicle during cornering.

[0002] Conventionally, various vehicle posture control apparatuses areknown, in which, such as in JP H06-183288A or JP H07-223520A, a targetyaw rate is set based on the steering wheel angle and the vehicle speed,the actual yaw rate of the vehicle is detected with a yaw rate sensor,and when the deviation between the detected actual yaw rate and thetarget yaw rate has at least a predetermined value, an understeercontrol suppressing an understeering tendency of the vehicle intervenesor an oversteer control suppressing an oversteering tendency of thevehicle intervenes.

[0003] More specifically, in such a posture control apparatus,understeer control intervenes when the target yaw rate is larger than avalue obtained by adding a predetermined threshold to the actual yawrate, whereas oversteer control intervenes when the actual yaw rate islarger than a value obtained by adding a predetermined threshold to thetarget yaw rate.

[0004] In this conventional posture control apparatus, too muchoversteer control tends to intervene when the threshold is small. Whenthe oversteer control intervenes too early, there is the problem thatthis invites difficulties in turning the vehicle due to excessivecontrol, and also the strength of the operation when intervention ofcontrol is unnecessary is a problem.

[0005] In order to rectify this, the threshold determining theintervention of the oversteer control should be increased, so that anearly intervention of the oversteer control can be inhibited. However inthat case, the control will intervene only when the oversteeringtendency of the vehicle is already too large. Moreover, when the controlfinally intervenes, a strong control intervenes abruptly. Therefore,even though it is possible to ensure the stability of the vehicle, in asituation in which the oversteering tendency keeps increasing until theoversteer control intervenes, the driver may feel that thecontrollability worsens, leading to a sense of instability.

[0006] In view of these problems, it is an object of the presentinvention to ensure a high vehicle stability and to enhance the sense ofstability and the controllability felt by the driver by improving theoversteer control.

SUMMARY OF THE INVENTION

[0007] In order to achieve these objects, according to a first aspect ofthe present invention, in addition to a first oversteer control forsuppressing an oversteering tendency, a second oversteer control isprovided, whose control amount is lower than that of the first oversteercontrol.

[0008] More specifically, the subject matter of the first aspect of thepresent invention is a vehicle posture control apparatus provided with acontrol means for controlling posture of the vehicle in yawing directionby independently controlling the brakes of the vehicle's wheels.

[0009] A special feature of the invention is that the control meanscarries out intervention of a first oversteer control suppressing theoversteering tendency when the oversteering tendency of the vehicle isstronger than a first preset reference value, and the control meanscarries out intervention of a second oversteer control, in which thecontrol amount is lower than in the first oversteer control, when theoversteering tendency of the vehicle is not stronger than the firstpreset reference value, but stronger than a second preset referencevalue.

[0010] Thus, when the oversteering tendency of the vehicle is strongerthan a first preset reference value, that is, when the oversteeringtendency is relatively strong, the first oversteer control intervenes tosuppress this oversteering tendency. This sufficiently ensures thestability of the vehicle.

[0011] Then, when the oversteering tendency of the vehicle is notstronger than the first preset reference value but stronger than thesecond preset reference value, or in other words, when there is arelatively weak oversteering tendency, intervention of the secondoversteer control, whose control amount is lower than that of the firstoversteer control, is carried out. Thus, by early intervention of thesecond oversteer control, this weak oversteering tendency is suppressed,and also growth of the oversteering tendency (the oversteering tendencybecoming stronger) is suppressed. Thus, the stability of the vehicle isincreased even more, improving the sense of stability felt by thedriver, while strong oversteering tendencies are suppressed, and theease of control felt by the driver is improved.

[0012] The second oversteer control is a weak control with a lowercontrol amount. Therefore, the second oversteer control does not lead toexcessive control even when it intervenes early, and unnecessaryoperation can be prevented from becoming too strong. As a result, anawkward feeling of the driver is prevented.

[0013] Furthermore, when for example the oversteering tendency increaseseven though the second oversteer control has intervened, and theoversteering tendency becomes stronger than the first preset referencevalue, then the first oversteer control intervenes, replacing the secondoversteer control. Thus, the first oversteer control, which has a strongcontrol amount, does not intervene abruptly. Also, the awkward feelingof the driver is largely rectified by continuously moving from thesecond oversteer control, which is a weak control, to the firstoversteer control, which is a strong control. Also, the second oversteercontrol intervenes previously to intervention of the first oversteercontrol, so that the play of the brake system is eliminated (forexample, leading to a state in which the brake pads adhere to the diskrotor). Therefore, the responsiveness of the first oversteer control isimproved. Furthermore, if the first oversteer control intervenes incontinuation to the intervention of the second oversteer control, theeffect is substantially the same as when lowering the threshold of theoversteer control, so that an even higher stability of the vehicle canbe ensured.

[0014] Consequently, providing the second understeer control separatelyfrom the first understeer control ensures a high stability of thevehicle while improving the sense of stability and the controllabilityfelt by the driver.

[0015] It is preferable that the second oversteer control supplies, byopen control, a brake pressure whose upper limit is a predeterminedbrake pressure that is lower than the maximum brake pressure that can besupplied in the first oversteer control.

[0016] That is to say, the responsiveness of the control is improved bysupplying the brake pressure by open control. At the same time, asuppression of an oversteering tendency with a control amount that islower than in the first oversteer control is achieved by setting apredetermined brake pressure that is lower than the maximum brakepressure that can be supplied in the first oversteer control. This way,a relatively weak oversteering tendency can be suitably adjusted withoutthe driver noticing the intervention of a control.

[0017] It is preferable that the second oversteer control supplies brakepressure in accordance with a deviation between a target yaw rate thathas been set and the actual yaw rate.

[0018] That is to say, by supplying the brake force by feedback controlin accordance with a deviation between a target yaw rate that has beenset and the actual yaw rate, there is no excessive suppression controlof the oversteering, and an optimal control is achieved. It should benoted that the upper limit of the brake pressure can also be set to apredetermined brake pressure that is lower than the brake pressureduring the first oversteer control. In this case, it is easy to achievea suppression control of oversteering, in which the control amount islower than in the first oversteer control.

[0019] It is preferable that the control means prohibits intervention ofthe second oversteer control when the vehicle has an understeeringtendency.

[0020] That is to say, if the second oversteer control would intervenewhen the vehicle is in an oversteering tendency while also being in anundersteering tendency, for example when the vehicle drifts out whilespinning, then the understeering tendency would be promoted. Therefore,when the vehicle has an understeering tendency, the intervention of thesecond oversteering tendency should be prohibited.

[0021] In a second aspect of the present invention, two oversteercontrols, namely a second and a third oversteer control, whose controlamounts are lower than that of the first oversteer control, are providedin addition to the first oversteer control for suppressing oversteeringtendencies.

[0022] More specifically, the subject matter of the second aspect of thepresent invention is a vehicle posture control apparatus provided with acontrol means for controlling posture of the vehicle in yawing directionby independently controlling brakes of the vehicle's wheels.

[0023] A special feature of the invention is that the control meanscarries out intervention of a first oversteer control suppressing anoversteering tendency when the oversteering tendency of the vehicle isstronger than a first preset reference value; the control means carriesout intervention of a second oversteer control, in which the controlamount is lower than in the first oversteer control, when theoversteering tendency of the vehicle is not stronger than the firstpreset reference value, but stronger than a second preset referencevalue; and the control means carries out intervention of a thirdoversteer control, in which the control amount is lower than in thefirst oversteer control, when the oversteering tendency of the vehicleis not stronger than the second preset reference value, but strongerthan a third preset reference value.

[0024] Thus, when the oversteering tendency of the vehicle is strongerthan a first preset reference value, the first oversteer controlintervenes to suppress this oversteering tendency. This suppressesrelatively strong oversteering tendencies of the vehicle.

[0025] Then, when the oversteering tendency of the vehicle is notstronger than the first preset reference value but stronger than thesecond preset reference value (when there is a relatively weakoversteering tendency), intervention of the second oversteer control,whose control amount is lower than that of the first oversteer control,is carried out.

[0026] Moreover, when the oversteering tendency of the vehicle is notstronger than the second preset reference value but stronger than thethird preset reference value (when there is an even weaker oversteeringtendency), intervention of the third oversteer control, whose controlamount is lower than that of the first oversteer control, is carriedout.

[0027] Thus, when the vehicle has a relatively weak oversteeringtendency, the second oversteer control intervenes early, suppressingthis oversteering tendency and suppressing an increase of theoversteering. As a result, the sense of stability as well as the ease ofcontrol felt by the driver are improved. The second oversteer controlintervening here is a weak control whose control amount is lower thanthat of the first oversteer control. Therefore, the control does notbecome excessive, and an unnecessary operation can be prevented frombecoming strong.

[0028] Furthermore, when the vehicle is in an even weaker oversteeringtendency, the third oversteer control intervenes early on. Thissuppresses the oversteering tendency and suppresses an increase of theoversteering. Thus, the sense of stability as well as thecontrollability felt by the driver are improved even more. Also thethird oversteer control is a weak control whose control amount is lowerthan that of the first oversteer control. Therefore, the control doesnot become excessive, and an unnecessary operation can be prevented frombecoming strong.

[0029] Furthermore, when the oversteering increases even though thethird oversteer control has intervened, then the second oversteercontrol intervenes, replacing the third oversteer control. And when theoversteering increases even more even though the second oversteercontrol has intervened, then the first oversteer control intervenes,replacing the second oversteer control. This way, the control movescontinuously from the third and the second oversteer control, which areweak controls, to the first oversteer control, which is a strongcontrol. As a result, the awkward feeling of the driver is largelyrectified. Furthermore, if the first oversteer control intervenes incontinuation to the intervention of the third and the second oversteercontrol, the effect is substantially the same as when lowering thecontrol threshold, so that an even higher stability of the vehicle canbe ensured.

[0030] Consequently, by providing the second and the second and thethird oversteer control separately from the first oversteer control, thesense of stability and the controllability felt by the driver areimproved even more, while ensuring a high stability of the vehicle.

[0031] It is preferable that a supply ratio of brake pressure during thethird oversteer control (amount of brake pressure supplied per unittime) is set to be lower than a supply ratio of brake pressure duringthe second oversteer control.

[0032] This prevents the control from becoming excessive when the thirdoversteer control intervenes during an extremely weak oversteeringtendency. As a result, a suitable suppression control of oversteeringtendencies can be achieved that is hardly noticed by the driver.

[0033] It is preferable that the second oversteer control supplies, byopen control, a brake pressure whose upper limit is a predeterminedbrake pressure that is lower than the maximum brake pressure that can besupplied in the first oversteer control.

[0034] Thus, as explained above, when the second oversteer controlintervenes during a relatively weak oversteering tendency, this weakoversteering tendency is suppressed without imparting an awkward feelingon the driver. Also, when the intervention of the first oversteercontrol succeeds the intervention of the second oversteer control, thenthe transition will be smooth.

[0035] It is preferable that the third oversteer control supplies brakepressure in accordance with a deviation between a target yaw rate thathas been set and the actual yaw rate.

[0036] Thus, the third oversteer control for suppressing extremely weakoversteering tendencies is suitably performed without becomingexcessive.

[0037] In addition, it is preferable that an upper limit of the brakepressure in the second and third oversteer control is set to 10 to 25bar.

[0038] By setting the upper limit of the brake pressure in this range,suitable posture control of the vehicle is achieved, even though thedriver will hardly notice it. It should be noticed that in order toachieve both the effect of controlling the posture of the vehicle andpreventing an awkward feeling due to the driver noticing theintervention of control, it is most preferable that the upper limit ofthe braking pressure is 15 bar.

[0039] Also in the second aspect of the present invention, it ispreferable that the control means prohibits intervention of the secondoversteer control when the vehicle has an understeering tendency.

[0040] It is further preferable that the control means prohibitsintervention of the third oversteer control when the vehicle has anundersteering tendency.

[0041] As mentioned above, if the second or third oversteer controlwould intervene when the vehicle is in an oversteering tendency whilealso being in an understeering tendency, then the understeering tendencywould be promoted. Therefore, when the vehicle has an understeeringtendency, the intervention of the second or third oversteering tendencyshould be prohibited.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a block diagram showing a vehicle posture controlapparatus.

[0043]FIG. 2A is a flowchart of a portion of the posture control.

[0044]FIG. 2B is a flowchart of a portion of the posture control.

[0045]FIG. 2C is a flowchart of a portion of the posture control.

[0046]FIG. 3 is a graph showing the amendment factor k as a function ofthe lateral acceleration.

[0047]FIG. 4 is a flowchart of the judgment regarding the start of thefirst understeer control.

[0048]FIG. 5 shows the relation between the actual yaw rate and thefirst target yaw rate, illustrating the conditions for the start of thefirst understeer control.

[0049]FIG. 6 shows the relation between the actual yaw rate and thefirst target yaw rate, illustrating the conditions for the start of thefirst understeer control, different from FIG. 5.

[0050] The upper diagram in FIG. 7 shows an example of the variation ofthe first target yaw rate, the second target yaw rate, the controltarget yaw rate and the actual yaw rate. The lower diagram in FIG. 7shows an example of the brake pressure supply in the first to thirdoversteer control.

[0051]FIG. 8 is a flowchart illustrating the convergence control aftercounter-steering.

[0052]FIG. 9 is a flowchart for setting the threshold value of the firstundersteer control.

[0053]FIG. 10 is a flowchart for setting the threshold value of thefirst oversteer control.

[0054]FIG. 11 illustrates the relation between the threshold of thefirst oversteer control and the vehicle speed.

[0055]FIG. 12 illustrates the amendment to the threshold of the firstoversteer control in accordance with the lateral acceleration and thevehicle speed.

[0056]FIG. 13 illustrates the overshooting of the actual yaw rate.

[0057]FIG. 14 is a flowchart illustrating the termination judgment ofthe first oversteer control.

[0058]FIG. 15 is a flowchart illustrating the brake pressure controlduring the first oversteer control and understeer control.

[0059]FIG. 16 is a flowchart illustrating the control of the warningapparatus.

DETAILED DESCRIPTION OF THE INVENTION

[0060]FIG. 1 shows the overall configuration of a vehicle posturecontrol apparatus in accordance with an embodiment of the presentinvention. First, the various devices on the input side shall beexplained. Numeral 11 denotes wheel speed sensors detecting the speed ofeach wheel. Numeral 12 denotes a steering angle sensor detecting thesteering angle of the steering wheel. Numeral 13 denotes a yaw ratesensor detecting the yaw rate of the vehicle. Numeral 14 denotes alateral acceleration sensor (lateral G-sensor) detecting theacceleration of the vehicle in a lateral direction. Numeral 15 denotes athrottle opening sensor detecting the throttle opening. Numeral 16denotes a stop lamp switch for canceling any control performed by theanti-lock brake system explained below. Numeral 17 denotes an enginespeed sensor detecting the engine speed. This engine speed sensor 17 isprovided in order to feedback control the engine output. Numeral 18denotes a shift position sensor (AT) detecting the shift position todetect the driving state of the engine (power train). This shiftposition detection sensor 18 is also used as a cancel switch forcanceling position control in case of reversing. Numeral 19 denotes anMC brake fluid pressure sensor detecting the brake fluid pressure of amaster cylinder (MC). Depending on the detection result of this MC brakefluid sensor 19, the brake fluid pressure is supplemented by a hydraulicpressure corresponding to the force with which the driver pushes downthe brake pedal. Numeral 110 denotes a reservoir brake fluid surfacelevel switch detecting whether there is brake fluid in the reservoir.

[0061] Next, the various devices on the output side shall be explained.Numeral 31 denotes an anti-lock brake system lamp indicating whether theabove-mentioned anti-lock brake system is operating. Numeral 32 denotesa pressurizing motor provided in a pressurizing pump. Numerals 33 and 34respectively denote a front solenoid valve 33 and a rear solenoid valve34 that are provided for the front wheels and the rear wheels, and thatsupply and draw away brake fluid to and from brake apparatuses made ofdisk brakes, for example. Numeral 35 denotes a TSW solenoid valve 35that opens and cuts off a passage between the master cylinder and thebrake apparatuses provided on each wheel. Numeral 36 denotes an ASWsolenoid valve that opens and cuts off a passage between the mastercylinder and the pressurizing pump mentioned above. Numeral 37 denotesan engine controller controlling the engine output. Numeral 38 denotes awarning apparatus serving as a warning means for informing the driveracoustically or visually when vehicle posture control is beingperformed.

[0062] The following is an explanation of an ECU 2 serving as a controlmeans, into which signals from the above-mentioned input-side sensorsand switches 11 to 110 are inputted, and which outputs control signalsto the above-mentioned output-side devices 31 to 38,

[0063] The ECU 2 includes an anti-lock brake system unit 21 forpreventing locking of the wheels by controlling the braking force whenthe wheels are about to lock with respect to the road surface, anelectronic braking force distribution device 22 that distributes thebraking force applied to the rear wheels so as to keep the rear wheelsfrom locking during braking, a traction control system 23 that preventsthe wheels from slipping with respect to the road surface by controllingthe driving force or the braking force applied to the wheels, and avehicle stabilizing control device 24 that controls the posture of thevehicle in the yaw direction, that is, in a drift-out or spinningdirection.

[0064] The following is an explanation of the input and output of thesignals into and from the various devices. A wheel speed calculatingunit and an estimated vehicle speed calculating unit calculate the wheelspeed of each wheel and the estimated vehicle speed, based on signalsreceived from the wheel speed sensors 11. The signal from the stop lampswitch 16 is inputted into a stop lamp status judging unit. The signalsfrom the wheel speed calculating unit, the estimated vehicle speedcalculating unit and the stop lamp status judging unit are inputted intothe anti-lock brake system unit 21, the electronic braking forcedistribution device 22, the traction control system 23 and the vehiclestabilizing control device 24.

[0065] The signals from the engine speed sensor 17, the throttle openingsensor 15 and the shift position sensor 18 are respectively inputtedinto an engine speed calculating unit, a throttle opening informationobtaining unit and a shift position judging unit, and from there intothe traction control system 23 and the vehicle stabilizing controldevice 24.

[0066] The signals outputted by the steering angle sensor 12, the yawrate sensor 13, the lateral G sensor 14, and the MC brake fluid pressuresensor 19 are respectively inputted into a steering angle calculatingunit, a yaw rate calculating unit, a lateral G calculating unit and anMC brake fluid pressure calculating unit, and based on these signals,the calculating units calculate the steering angle, the yaw rate, thelateral acceleration, and the MC brake fluid pressure, which are theninputted into the vehicle stabilizing control device 24.

[0067] The signal from the reservoir brake fluid surface level switch110 is inputted into the traction control system 23 and the vehiclestabilizing control device 24 via a brake fluid surface level judgingunit.

[0068] The anti-lock brake system 21 calculates control values based onthe inputted signals, and outputs signals to the anti-lock brake systemlamp 31, the pressurizing motor 32, the front solenoid valve 33 and therear solenoid valve 34 to control these components.

[0069] The electronic braking force distribution device 22 controls therear solenoid valve 34.

[0070] The traction control system 23 outputs signals to the frontsolenoid valve 33, the rear solenoid valve 34, the pressurizing motor32, the TSW solenoid valve 35, and the engine controller 37 to controlthese components.

[0071] The vehicle stabilizing control device 24 outputs signals to theengine controller 37, the front solenoid valve 33, the rear solenoidvalve 34, the pressurizing motor 32, the TSW solenoid valve 35, the ASWsolenoid valve 36, and the warning apparatus 38 to control the operationof these components.

Vehicle Posture Control

[0072] The following is an explanation of the vehicle posture controlwith the vehicle stabilizing control device 24. The vehicle stabilizingcontrol device 24 performs understeer control, in which for exampledrifting out is avoided or suppressed, and oversteer control, in whichfor example spinning is avoided or suppressed. For understeer control,three control modes, namely a first understeer control, a secondundersteer control and engine control are provided, and also for theoversteer control, three control modes, namely a first, second and thirdoversteer control are provided.

[0073] The first understeer control is a relatively strong control (theposture change of the vehicle is relatively large), in which a brakingforce is applied to the front wheel situated on the inside of the turn(cornering inside front wheel) or the rear wheel situated on the insideof the turn (cornering inside rear wheel) when the deviation between acontrol target yaw rate Tr ø and the actual yaw rate ø is larger than apredetermined intervention threshold. In engine control, the engineoutput is lowered when the deviation between a control target yaw rateTr ø and the actual yaw rate ø is larger than a predeterminedintervention threshold. With these two types of control, the centrifugalforce is lowered due to the decrease of vehicle speed, and a momentumacts on the vehicle due to the unbalance of the braking forces appliedto the wheels. As a result, drifting out can be avoided or suppressed.By contrast, in the second understeer control, a braking force with acontrol amount that is smaller than for the first understeer control isapplied on the cornering inside front wheel when the actual yaw ratedoes not undergo a predetermined change in response to a change of thesteering wheel angle of the vehicle. Thus, it is possible to suppressrelatively week understeering tendencies and the growth ofundersteering, and it is possible to keep the driver from feeling thatthe vehicle tends to understeer.

[0074] On the other hand, the first oversteer control is a relativelystrong control, in which a braking force is applied to the front wheelsituated on the outside of the turn (cornering outside front wheel) whenthe deviation between a control target yaw rate Tr ø and the actual yawrate ø is larger than a predetermined intervention threshold. With thiscontrol, a momentum is generated that pushes the front of the vehicleoutward with respect to the cornering direction, so that spinning can beavoided or suppressed. By contrast, in the second oversteer control, thecontrol intervention threshold is smaller than the control interventionthreshold of the first oversteer control, and in the third oversteercontrol, the control intervention threshold is even smaller than thecontrol intervention threshold of the second oversteer control. Both thesecond and the third oversteer control are weaker than the firstoversteer control (that is, the posture change of the vehicle issmaller). In the second oversteer control, brake pressure is suppliedwith open control in which a predetermined brake pressure is taken as anupper limit, whereas the third oversteer control is a feedback control,in which brake pressure is supplied depending on the deviation betweenthe control target yaw rate Tr ø and the actual yaw rate ø.

[0075] The following is a more detailed explanation of the posturecontrol with the vehicle stabilizing control device 24, with referenceto the flowchart in FIG. 2. First, in Step S11, signals are read in fromthe sensors and switches numbered 11 to 110.

[0076] In step S12, a first target yaw rate ø(θ) is calculated inaccordance with the steering angle, and a second target yaw rate ø(G) iscalculated in accordance with in accordance with the lateralacceleration.

[0077] More specifically, the first target yaw rate ø(θ) is calculatedusing Equation (1) below, using (a) the estimated vehicle speed V thathas been calculated by the estimated vehicle speed calculating unitbased on the signals outputted by the wheel speed sensors 11 and (b) thesteering angle θ that has been detected by the steering angle sensor 12and calculated by the steering angle calculating unit.

ø(θ)=V×θ/{(1+K×V ²)×L}  (1)

[0078] In Equation (1), K represents a stability factor for the vehicle,which is a constant that is determined from a cornering field on a highμ(friction factor) road. L is the wheelbase.

[0079] The second target yaw rate ø(G) is calculated with Equation (2)below, using the estimated vehicle speed V described above and thelateral acceleration Gy that has been calculated by the lateral Gcalculating unit based on the signal from the lateral G sensor 14.

ø(G)=Gy/V  (2)

[0080] Next, in step S13, it is judged whether the absolute value of thesecond target yaw rate ø(G) is below the absolute value of the firsttarget yaw rate ø(θ). This judgment is performed as a step for decidingwhich of the first target yaw rate ø(θ) and the second target yaw rateø(G) should be set as the control target yaw rate Tr ø. Out of the firstand second target yaw rate ø(θ, G), the one with the smaller absolutevalue is set as the target yaw rate Trø.

[0081] When the judgment at Step S13 is “NO”, then the procedureadvances to Step S14, and when it is “YES”, then the procedure advancesto Step S15.

[0082] At Step 514, the first target yaw rate ø(θ) is taken as thecontrol target yaw rate Tr ø and the deviation Δø(θ) to the actual yawrate ø detected with the yaw rate sensor 13 and calculated with the yawrate calculating unit is calculated.

[0083] On the other hand, at Step S15, the second target yaw rate ø(G)is taken as the control target yaw rate Tr ø. Herein, the control targetyaw rate Tr ø is amended with a steering angle component, as shown inEquation (3):

Tr ø=ø(G)+a×k 1  (3)

[0084] In Equation (3), a=ø(θ)−ø(G), and k1 is a variable.

[0085] Then, the deviation Δø(θ, G) between this amended control targetyaw rate Tr ø and the actual yaw rate ø is calculated.

[0086] Thus, when the second target yaw rate ø(G) based on lateralacceleration is taken as the control target yaw rate Tr ø, the amendmentwith the steering angle component makes it possible to suppress theintervention of posture control when the driver intentionally provokesan understeering tendency (so-called “intentional understeering”).

[0087] This is to say, there are two kinds of understeer. Intentionalundersteering is intentionally caused by the driver in an operationwhere the driver keeps the steering wheel at a constant angle andsimultaneously increases the driving force when the vehicle tends toundersteer. Unintentional understeering is caused when the vehiclebehavior cannot follow the driver's operation of the steering wheel.When the control target yaw rate Tr ø is set using the second target yawrate ø(G) that is based on the lateral acceleration, then the lateralacceleration on the vehicle is the same for both types of understeer.For that reason, posture control will also be performed for theabove-described intentional understeering. In order to avoid this, thesteering angle component is amended when the second target yaw rate ø(G)is taken as the control target yaw rate, so that posture control isperformed only when the driver has turned the steering wheel. As aresult, posture control is not controlled during intentionalundersteering, but only when the driver understeers intentionally.

[0088] For k1 in Equation (3), a value is chosen that changes dependingon the lateral acceleration, for example as shown in FIG. 3. That is tosay, when the lateral acceleration is small (in regions where the roadsurface has a low μ, such as on an icy surface) or when the lateralacceleration is large (in regions with a high μ), a small value ischosen for k1, and the amendment ratio of the steering angle componentis low.

[0089] The reason for this is that when a large k1 is chosen in regionswith a low μ for example, the following problem occurs: In regions withlow μ, there tends to be a lesser response to turning the steeringwheel, so that the driver usually turns the steering wheel atcomparatively large steering angles. In this case, if the amendmentratio of the steering angle component is large because k1 has been setto a large value, then the deviation between the control target yaw rateTr ø and the actual yaw rate ø becomes large, and the control amount ofthe posture control, such as the braking amount, becomes large as well.As a result, the vehicle behavior after posture control has beenperformed will be too large with respect to the opposite direction, andthere is the risk that it is difficult to rectify this behavior withrespect to the opposite direction.

[0090] Moreover, the reason why k1 is set to a small value in regionswith high μ is that when a large value is chosen for k1 to set a largesteering angle component while a sufficient gripping force of the tirescan be attained, then the posture control starts too early. That is tosay, in regions with high μ, suitable control intervention is realizedeven when the amendment ratio for the steering wheel component is notlarge, so that in regions with high μ, k1 is set to a small value.

[0091] On the other hand, a lateral acceleration at a medium level(region with medium μ) corresponds to the road surface μ of a roadcovered with packed snow, and there is a large possibility ofsideslipping. Setting k1 to a large value at medium levels of lateralacceleration, the amendment ratio of the steering angle component islarge, so that the posture control is performed at an early stage.

[0092] Thus, changing the value of k1 depending on the lateralacceleration ensures that posture control intervenes at suitable timing.

[0093] Then, at Steps S14 and Step S15, the deviation Δø(θ, G) betweenthe control target yaw rate Tr ø and the actual yaw rate ø iscalculated, and the procedure advances to Step S16. Step S16 sets thethreshold deciding whether the first oversteer control is performed(first intervention threshold: THOS), the threshold deciding whetherengine control for suppressing understeer is performed (THEUS), thethreshold deciding whether the first understeer control is performed(THUS), the threshold deciding whether the second oversteer control isperformed (second intervention threshold: THOSII), and the thresholddeciding whether the third oversteer control is performed (thirdintervention threshold: THOSIII). It should be noted that THUS>THEUS.Furthermore, THOSII and THOSIII are set such that THOSII<THOS andTHOSIII<THOSII. For example, it is possible to set THOSII to be about10% smaller than THOS, and THOSIII to be about 20% smaller than THOS. Itis also possible to set THOSII to be about 20% smaller than THOS, andTHOSIII to be about 30% smaller than THOS.

[0094] When all thresholds have been set at Step S16, the procedureadvances to Step S17.

Second Understeer control

[0095] Steps S17 to S110 are the steps for the second understeercontrol. First, at Step S17, it is judged whether the steering wheel hasbeen turned in a situation in which the vehicle is moving straightforward, and whether the deviation Δø between the control target yawrate Tr ø and the actual yaw rate ø is smaller than the thirdintervention threshold THOSIII (Δø<THOSIII). That is to say, when thesteering wheel has been turned in a situation in which the vehicle ismoving straight forward, the purpose of the second understeer control isto suppress a weak understeering tendency at the beginning of a turningof the steering wheel when the increase of the lateral G is small and itis difficult to detect the lateral G, as well as to suppress a situationin which the driver feels that there is an understeering tendency. Forthis reason, it is judged whether the steering wheel has been turned ina situation in which the vehicle is moving straight forward.

[0096] The reason why it is judged whether the yaw rate deviation Δø(θ,G) is smaller than the third intervention threshold THOSIII is asfollows: When there is an oversteering tendency, in which the yaw ratedeviation Δø is equal to or greater than the third threshold THOSIII,then it is first necessary to suppress this oversteering tendency. Ifthe second understeer control for suppressing an understeering tendencyintervenes in this situation, then there is the risk that it promotesthe oversteering tendency. Therefore, in order not to intervene with thesecond understeer control, it is judged whether the yaw rate deviationΔø(θ, G) is smaller than the third intervention threshold THOSIII.

[0097] At Step S17, if Δø<THOSIII and the steering wheel has been turnedin a situation in which the vehicle is moving straight forward, then theprocedure advances to Step S18, whereas if the steering wheel has notbeen turned in a situation in which the vehicle is moving straightforward or if Δø≧THOSIII, then the procedure advances to Step S111 (seeFIG. 2B) without performing the second understeer control.

[0098] Step S18 judges whether the value of {change ratio of firsttarget yaw rate ø(θ)} {change ratio of the actual yaw rate ø} hasincreased positively, or in other words, whether the actual yaw rate ødoes not make a predetermined change with respect to the change of thefirst target yaw rate ø(θ) (following without changing) and ø(θ) and øare moving away from one another. This judges whether in this situationthe driver can feel an understeering tendency (a situation in which theactual yaw rate does not follow the change of the steering wheel), andif the first target yaw rate ø(θ) and the actual yaw rate ø have movedaway from one another, it is judged whether in this situation there is astrong understeering tendency (initial understeering situation). If theresult of the judgment is YES, then the procedure advances to Step S19,and if it is NO, then the procedure advances to Step S111.

[0099] Step S19 is the step in which the second understeer controlintervenes. The upper limit of the brake pressure is set to 30 bar, andbrake pressure is supplied to the cornering inside front wheel at thebrake pressure gain of Kmax. Here, the brake pressure gain Kmax is themaximum gain (maximum brake pressure supply ratio (brake pressure supplyamount per unit time)). However, when brake pressure is supplied at thegain Kmax, the supply of brake pressure is stopped when slippageincreases or when the turning of the steering wheel is terminated.

[0100] Then at Step S110, if the value of {change ratio of first targetyaw rate ø(θ)} {change ratio of the actual yaw rate ø} that tended toincrease has been switched to a decreasing tendency, then the brakepressure is reduced. If {change ratio of first target yaw rate ø(θ)}{change ratio of the actual yaw rate ø} does not start to decrease, thenthe brake pressure is held without decreasing. With this control, thesupply pattern of the brake pressure over time becomes trapezoid.

[0101] Thus, by intervention of the second understeer control at aninitial state of understeering, separately from the first understeercontrol, strong understeering tendencies of the vehicle can besuppressed. Thus, the sense of stability felt by the driver can beimproved. Moreover, by intervention of the second understeer control ina situation when the driver feels an understeering tendency, the postureof the vehicle is changed in a direction intended by the driver.Therefore, the controllability felt by the driver can be improved.

[0102] Moreover, in the second understeer control, the upper limit ofthe brake pressure is set to a relatively low pressure of 30 bar, and if{change ratio of first target yaw rate ø(θ)} {change ratio of the actualyaw rate ø} starts to decrease, the control is stopped. Therefore, eventhough the posture of the vehicle changes slightly in response to thesteering of the driver due to the intervention of the second understeercontrol, this posture change is not large. When intervention of such aweak second understeer control, the driver feels that the behavior ofthe vehicle follows the driver's steering, and will hardly feel thatcontrol has intervened. As a result, an awkward feeling of the drivercaused by the intervention of the control can be prevented, while thedriving experience can be improved.

[0103] Moreover, the second understeer control intervenes when asufficient yaw rate change with respect to the steering cannot beattained, when the steering wheel has been turned in a situation inwhich the vehicle is moving straight forward. Thus, the necessary yawrate change can be attained near the entry into a cornering path. As aresult, a situation can be avoided, in which a small cornering radius Rmust be taken near the exit of the cornering path. That is to say, it ispossible to improve the tracing properties with respect to a targetcornering trace.

[0104] Furthermore, in the second understeer control, the corneringinside front wheel is subjected to braking, so that it is effective tosuppress an understeering tendency, and understeering can be suppressedreliably and fast.

[0105] Thus, the second understeer control is particularly useful at thebegin of cornering when the intervention of the first understeer controlwith the second target yaw rate ø(G) based on the lateral G is difficultbecause the increase of the lateral G is small and the detection of thelateral G with the lateral G sensor is difficult. By intervention ofthis second understeer control with a reduced control amount at acomparatively early stage based on the steering wheel angle (firsttarget yaw rate Δ(θ)) and the actual yaw rate ø, it can be preventedthat the understeering tendency is suppressed too late. At the sametime, it can be avoided that the strong first understeer controlintervenes abruptly. And what is more, the second understeer control isa control with a reduced control amount and a control in which thevehicle posture is modified in the direction intended by the driver.Therefore, even when the second understeer control intervenes early on,the driver will hardly notice that a control has intervened.Consequently, a high vehicle stability can be ensured and an awkwardfeeling of the driver can be prevented, while the sense of stability andcontrollability felt by the driver can be improved.

Engine Control

[0106] The Steps S11 to S118 following the steps of the secondundersteer control are the steps related to the engine control forsuppressing an understeering tendency.

[0107] First, in Step S111, it is judged whether the above-mentionedTHEUS is larger than the deviation Δø(θ) between the first target yawrate ø(θ) and the actual yaw rate ø. That is to say, it is judgedwhether engine control should be performed.

[0108] To judge whether engine control should be performed, the value ofthe first target yaw rate ø(θ) is taken as the reference, even when thesecond target yaw rate ø(G) has been selected as the target yaw rate inStep S13.

[0109] This is due to the following reason: If the first target yaw rateø(θ) is taken as the control target yaw rate Tr ø to perform the posturecontrol, then, since the phase of the steering angle signal is fast, theposture control is usually started early on. Therefore, in thisembodiment, early intervention of the posture control (first understeercontrol) is prevented by using both the first and the second target yawrate. It should be noted that there is little damage in starting onlythe engine control early, because the driver will notice a decrease ofthe engine output less often than braking control.

[0110] Furthermore, first decelerating the vehicle is useful in order toavoid an understeering tendency, and if the vehicle is decelerated forthis reason by reducing the engine output early on, then understeeringcan be avoided effectively.

[0111] Moreover, because of the substantially proportional relationbetween lateral acceleration and yaw rate, there is no large differencebetween the value ø(G) of the second target yaw rate based on thelateral acceleration and the actual yaw rate ø. Moreover, the actual yawrate ø becomes instable in the case of an understeering tendency, sothat if the second target yaw rate ø(G) is taken as the control targetyaw rate Tr ø, the correct intervention of control becomes difficult.For these reasons, the first target yaw rate ø(θ) is taken as thecontrol target yaw rate Tr ø for the judgment regarding the start ofengine control.

[0112] Then, if the judgment at Step S111 is “YES”, then the procedureadvances to Step S112, and if the judgment is “NO”, then the procedureadvances to Step S113, and it is judged whether the first oversteercontrol should be started.

[0113] Step S112 judges whether the yaw rate acceleration is below apredetermined value. The purpose of this is to prevent erroneousintervention of a control, and it is judged whether the vehicle issubject to a posture change of at least a predetermined amount. Then, ifthe judgment is “YES”, then the procedure advances to Step S114, and ifthe judgment is “NO”, then the procedure advances to Step S117, theengine control is prohibited, and the procedure advances to Step S113.

[0114] Step S114 judges whether the vehicle is currently oversteering.This is because a situation is conceivable in which the vehicle movesout of the cornering path while rotating in the cornering direction, orin other words, an oversteering tendency and an understeering tendencyoccur simultaneously. In this situation, it is foremost necessary torectify the posture of the vehicle by avoiding the oversteeringtendency. If the result of the judgment is “YES”, then the procedureadvances to Step S117, where the engine control is prohibited, and thenthe procedure advances to Step S113. If, on the other hand, the judgmentis “NO”, then the procedure advances to Step S115.

[0115] Step S115 judges whether the brakes are currently released ornot. This is because, if the driver is operating the brakes, then notonly is no driving power generated and the effect of the engine controlis small, but when engine control is performed and then the acceleratorpedal is pressed down, then it is not possible to accelerate. Thus, inorder not to perform unnecessary engine control, the procedure advancesto Step S117 where engine control is prohibited, if the driver isoperating the brakes. On the other hand, if the judgment is “YES”, thenthe procedure advances to Step S116, and the engine suppression controlamount for the engine control is calculated. Then, the procedureadvances to Step S118, and engine control is performed by outputting asignal to the engine controller 37, that is to say, the engine output isreduced. After Step S118 has been finished, the procedure advances toStep S113.

First Oversteer Control

[0116] Steps S113 and S119 to S121 are the steps related to the firstoversteer control. In Step S113, it is judged whether the firstoversteer control should be carried out or not. This judgment regardingthe first oversteer control is carried out by judging whether the yawrate deviation Δø(θ, G) calculated in Steps S14 and S15 is larger thanthe first intervention threshold THOS. That is to say, the oversteeringtendency expressed by the yaw rate deviation Δø(θ, G) is judged,depending on whether it is stronger than the first set referenceexpressed by the first intervention threshold THOS. If the judgment is“YES”, then the procedure advances to Step S119, and the braking forceapplied to the outer front wheel for which the oversteering tendency isto be rectified, in other words the front wheel on the outer side withrespect to the yaw rate rotation direction, is set in accordance withthe yaw rate deviation Δø(θ, G).

[0117] When the braking amount has been set, the procedure advances toStep S120, and the braking force control is carried out. This is done bycontrolling the pressurizing motor 32, the front and rear solenoids 33and 34, and the TSW and ASW solenoids 35 and 36 (see lower diagram inFIG. 7). Then, the procedure advances to Step S121, a terminationjudgment of the first oversteer control is performed, and the procedurereturns. This termination judgment is explained in detail below.

Second and Third Oversteer Control

[0118] If the result of the judgment at Step S113 is “NO”, then theprocedure advances to Step S122. The Steps S122 to S127 are the stepsrelated to the second and third oversteer control.

[0119] At Step S122, it is judged whether the second oversteer controlshould be carried out. This judgment regarding the second oversteercontrol is carried out by judging whether the yaw rate deviation Δø(θ,G) set at Step S14 or Step S15 is larger than the second interventionthreshold THOSII, that is THOSII<Δø. That is to say, it is judgedwhether the oversteering tendency expressed by the yaw rate deviationΔø(θ, G) is stronger than the second set reference expressed by thesecond intervention threshold THOSII. If THOSII<Δø, in other words ifthe judgment is “YES”, then the procedure advances to step S123, and ifTHOSII>Δø, in other words if the judgment is “NO”, then the procedureadvances to Step S124.

[0120] Step S123 is the step at which the second oversteer controlsuppressing a relatively weak oversteering tendency intervenes, andbrake pressure is supplied at once to the cornering outside front wheelat the gain Kmax with a brake pressure of P2 (15 bar) as the upper limit(see lower diagram in FIG. 7). Then, when the yaw rate deviation Δøwhile supplying brake pressure has decreased, the supply of brakepressure is stopped, and the brake pressure is changed to reducedpressure. Consequently, when the yaw rate deviation Δø has increased, abrake pressure of up to the upper limit brake pressure P2 is supplied.

[0121] At the next Step S126, a termination judgment of the secondoversteer control is performed. That is to say, Step S126 judges whetherΔø has converged or not (whether Δø has been decreased or not). If Δøhas been decreased by intervention of the second oversteer control (i.e.“YES”), the procedure advances to Step S127, the control is graduallybrought to an end, and the procedure returns. On the other hand, if Δøhas not converged (i.e. “NO”), then the procedure returns withoutadvancing to Step S127, and the second oversteer control is continued.

[0122] If at Step S122 THOSII≧Δø(i.e. “NO”) and the procedure hasadvanced to Step S124, then Step S124 judges whether the third oversteercontrol should be carried out. This judgment regarding the interventionof the third oversteer control is performed by judging whetherTHOSII<Δø. That is to say, it is judged whether the oversteeringtendency expressed by the yaw rate deviation Δø(θ, G) is stronger thanthe third set reference expressed by the third intervention thresholdTHOSIII. If THOSII<Δø, in other words if the judgment is “YES”, then theprocedure advances to Step S125, and if THOSII≧Δø, in other words if thejudgment is “NO”, then the procedure advances to Step S128 (see FIG.2C).

[0123] In this Step S125, at which the third oversteer controlintervenes, brake pressure is supplied at once to the cornering outsidefront wheel at the brake pressure gain Kmax with an upper limit brakepressure (hydraulic) of P1 (5 bar). After that, a feedback control iscarried out, in which brake pressure is supplied in accordance with Δøat a gain K₁ (K₁<Kmax). In this situation, the upper limit of the brakepressure is set to P2 (15 bar) (see lower diagram in FIG. 7). Thus, inthe third oversteer control, brake pressure is supplied at the brakepressure gain K₁, so that the supply ratio of the brake pressure islower than the supply ratio (gain Kmax) of the brake pressure in thesecond oversteer control.

[0124] After the brake pressure has been supplied at Step S125, thetermination judgment is carried out at Step S126, and if Δø isconverging (i.e. “YES”) , the procedure advances to Step S127, and thecontrol is gradually terminated. On the other hand, if Δø is notconverging (i.e. “NO”), then the procedure returns without advancing toStep S127, and the third oversteer control is continued.

[0125] Thus, if the oversteering tendency is relatively weak (THOSII,THOSIII<Δø), then the relatively weak oversteering tendency and theincrease of the oversteer are both suppressed by the intervention of thesecond or the third oversteer control, and that the sense of stabilityas well as the ease of maneuvering felt by the driver are improved.

[0126] On the other hand, the intervening second and the third oversteercontrols are weak controls in which the upper limit of the brakepressure is set to be low, thus decreasing the control amount.Therefore, the control does not become excessive, and it can be avoidedthat an unnecessary operation becomes too strong.

[0127] Moreover, if the oversteering of the vehicle has increased(become stronger) even though the third oversteer control has intervened(THOSII<Δø), then the second oversteer control intervenes, replacing thethird oversteer control. Furthermore, if the oversteering of the vehiclehas increased even though the second oversteer control has intervened(THOS<Δø), then the first oversteer control intervenes, replacing thesecond oversteer control. Thus, the strong first oversteer control doesnot intervene abruptly, but the system moves continuously from theweaker second and third controls to the strong first oversteer control.Thus, abruptly intervention of the strong first oversteer control causesan awkward feeling of the driver, but this awkward feeling can beeliminated. At the same time, by intervention of the second or the thirdoversteer control before the first oversteer control, the play of thebrake system is eliminated (leading to a situation in which break padsadhere on the disk rotor). Therefore, the responsiveness of the firstoversteer control can be improved. Furthermore, if the first oversteercontrol intervenes in continuation of the third and the second oversteercontrol, then the same result is attained as if the threshold forstarting control is reduced, so that changes in the posture of thevehicle become continuous and an even better stability of the vehiclecan be ensured.

[0128] Furthermore, in the second oversteer control, brake pressure issupplied by an open control at the maximum gain Kmax. Therefore, theresponsiveness of the control is improved. Moreover, the upper limit ofthe brake pressure P2 is set to a pressure (15 bar) that is lower thanthe brake pressure (brake pressure that can be supplied by the brakesystem) for the first oversteer control, so that an oversteer controlcan be achieved, in which the control amount is lower than in the firstoversteer control.

[0129] On the other hand, in the third oversteer control, the brakepressure is supplied by a feedback control depending on the yaw ratedeviation Δø. Therefore, the suppression of the oversteering tendencydoes not become excessive, and it is possible to achieve the optimumcontrol. As a result, the driving experience is not disturbed.

[0130] Moreover, by setting the upper limit of the brake pressure P2 inthe second and the third oversteer control to 15 bar, it is possible tokeep the controls at a level that is hardly noticed by the driver, eventhough the posture of the vehicle in yawing direction changes slightlyIt should be noted that the upper limit of the brake pressure can be setwithin a range of 10 to 25 bar, but in order to both achieve control ofthe posture of the vehicle and prevent an awkward feeling of the drivercaused by the driver noticing the control intervention, it is mostpreferable that the upper limit of the brake pressure is set to 15 bar.Moreover, it is also possible to modify the upper limit of the brakepressure at the second oversteer control in accordance with the roadsurface μ. For example, it is possible to set the maximum brake pressureto 15 bar on roads with a low μ and to 50 bar on roads with a high μ.

[0131] Thus, by providing a second and a third oversteer control inaddition to the first oversteer control, it is possible to improve thesense of stability and the ease of control felt by the driver, whileensuring a high stability of the vehicle.

First Understeer Control

[0132] Steps S128 to S134 are the steps related to the first understeercontrol. If the result of the judgment at Step S124 is “NO”, and theprocedure advances to Step S128, then Step S128 judges whether the firstundersteer control should be started or not. This judgment is explainedin detail below. If Step S128 judges that control should be started(i.e. “YES”), then the procedure advances to Step S129, and if Step S128judges that control should not be started (i.e. “NO”), then theprocedure returns.

[0133] At Step S129, it is judged whether the understeering tendency isweak. If it is weak, then the procedure advances to Step S130, and if itis strong, then it advances to Step S131.

[0134] At Step S130, the braking amount for the inner front wheel iscalculated. This is, because when the understeering tendency is weak, itis likely that the front wheels have gripping power. Also, applying abraking force to the front wheels, the braking efficiency is better thanwhen applying a braking force to the rear wheels, which means that thevehicle can be decelerated more effectively. Therefore, if theundersteering tendency is weak, understeer control can be performedreliably and fast by braking the inner front wheel.

[0135] On the other hand, Step S131 calculates the braking amount forthe inner rear wheel. This is, because when the understeering tendencyis strong, it is likely that the front wheels have no gripping power.Therefore, when the understeering tendency is strong, the braking forceis applied to the inner rear wheel.

[0136] If the braking amount has been calculated in this manner, theprocedure advances to Step S132, and the control of the braking power iscarried out.

[0137] Then, at Step S133, a termination judgment of the firstundersteer control is carried out. This is done by judging whether theyaw rate deviation Δø(θ, G) has become smaller than the threshold THUS.If the result of this judgment is “YES”, then the procedure advances toStep S134, the control is terminated, and the procedure returns. On theother hand, if it is “NO”, then the procedure returns withoutterminating the procedure.

Judgment Regarding Start of the First Understeer Control

[0138] Referring to the flowchart in FIG. 4, the following is anexplanation of the judgment at Step S128 regarding the start of thefirst understeer control for suppressing an understeering tendency. Inthis judgment regarding the start of the control, it is not only judgedwhether the yaw rate deviation Δø(θ, G) has exceeded a threshold THUS,but measures are taken so that the control is started depending on otherconditions as well.

[0139] First, Step S21 judges whether the yaw rate deviation Δø(θ, G) islarger than the intervention threshold of the first understeer control.If the judgment is “YES”, then the procedure advances to Step S22, andif it is “NO”, then the procedure advances to Step S23.

[0140] At this Step S22, it is judged whether the acceleration of theactual yaw rate ø is below a predetermined value. This is done for thesame purpose as in the afore-mentioned Step S112 (see FIG. 2B), namelyto prevent erroneous intervention of a control.

[0141] Then, Step S23 judges whether the speed of the steering wheeloperation in the direction increasing the turning (i.e. decreasing thecurve radius) has at least a predetermined value. If the judgment is“YES”, then the procedure advances to Step S25, and if it is “NO”, thenthe procedure advances to Step S27, and the procedure returns withoutperforming control. Then, Step S25 judges whether, as shown in FIG. 5,the first target yaw rate ø(θ) is two times larger than the actual yawrate ø, and whether the value Δø(θ) of {first target yaw rate ø(θ)actual yaw rate ø} is at least a predetermined value. If the judgment atStep S25 is “NO”, then the procedure advances to Step S26, and it isjudged whether, as shown in FIG. 6, the acceleration of the actual yawrate ø is not greater than a predetermined value, and whether Δø(θ) hasat least a predetermined value. If the judgment is “NO”, then theprocedure advances to Step 527, and the procedure returns withoutperforming control.

[0142] Step S25 judges whether the deviation between the first targetyaw rate ø(θ) and the actual yaw rate ø is large, whereas Step S26judges whether the spread of the deviation between the first target yawrate ø(θ) and the actual yaw rate ø is fast. If the judgment at Step S25or at Step S26 is “YES”, then the procedure advances to Step S24, andthe braking control begins.

[0143] That is to say, when the posture control is started based only onwhether the yaw rate deviation Δø(θ, G) is larger than the thresholdTHUS, the control will also be started when the driver intentionallycauses an understeering tendency, as in intentional understeering. Forthis reason, the control is carried out only when the steering wheel isturned but the yaw rate does not increase accordingly, or in otherwords, when the vehicle tends to understeer while not acting as intendedby the driver.

Judgment Regarding Start of Oversteer Control

[0144] The following is an explanation of the judgment regarding thestart of the oversteer control. As mentioned above, in the judgmentsregarding the start of the first to third oversteer control, of thefirst and the second target yaw rate ø(θ, G), the one with the smallerabsolute value is taken as the control target yaw rate Tr ø, and thejudgment is carried out by judging whether the deviation Δø(θ, G)between the control target yaw rate Tr ø and the actual yaw rate ø islarger than the intervention thresholds (first to third interventionthresholds) THOS, THOSII and THOSIII of the oversteer control.

[0145] For example, when the absolute value of the second target yawrate ø(G) is smaller than the absolute value of the first target yawrate ø(θ) as shown in FIG. 7, then the second target yaw rate ø(G) istaken as the control target yaw rate Tr ø(see T1 in FIG. 7). Here, thereason why the control target yaw rate Tr ø(see broken curve in FIG. 7)is larger than the second target yaw rate ø(G) (solid line in FIG. 7) isthat the control target yaw rate Tr ø is amended by the steering anglecomponent (see Equation (3)).

[0146] Then, when the yaw rate deviation Δø has become larger than thethird intervention threshold THOSIII, the third oversteer controlintervenes. As explained above, during this third oversteer control,brake pressure is supplied at once with the brake pressure gain Kmax atup to an upper limit brake pressure P1 (5 bar). After that, a feedbackcontrol is performed, in which the brake pressure is supplied at a gainof K₁ (K₁<Kmax) accordance with Δøø (see Step S125 in FIG. 2B and lowerdiagram in FIG. 7). Moreover, when the yaw rate deviation Δø has becomelarger than the second intervention threshold THOSII, the secondoversteer control intervenes. As explained above, during this secondoversteer control, an open control is performed, in which the brakepressure is supplied at once at the gain of Kmax at up to a maximumbrake pressure P2 (15 bar) (see Step S123 in FIG. 2B and lower diagramin FIG. 7). Moreover, if the yaw rate deviation Δ ø becomes larger thanthe first intervention threshold THOS, the first oversteer controlintervenes.

[0147] If for example the driver performs counter-steering in order torectify the oversteering tendency, then the value of the first targetyaw rate ø(θ) may become smaller than the second target yaw rate ø(G).In this situation, the control target yaw rate Tr ø is changed from thesecond target yaw rate ø (G) to the first target yaw rate ø(θ) (see T2in FIG. 7).

[0148] When counter-steering has been performed in this manner, then,following the change of the first target yaw rate ø(θ), the value of theactual yaw rate becomes smaller than the second target yaw rate ø(G). Inthis situation, if the control target yaw rate Tr ø is still set to thesecond target yaw rate ø(G), then the oversteer control switches toundersteer control. Switching to understeer control in this manner,leads to a control in which the posture of the vehicle in yawingdirection is still in an oversteering tendency, and even though thedriver tries to counter-steer, this counter-steering is without effect,and the oversteering tendency is promoted. If, by contrast, the smallerone of the first and the second target yaw rate ø(θ, G) is taken as thecontrol target yaw rate Tr ø, then, even when counter-steering isperformed, the oversteer control (first oversteer control) will becontinued, and this problem is avoided.

[0149] When the first target yaw rate ø(θ) passes the neutral point, andthe sign of the first target yaw rate ø (θ) is different from the signof the second target yaw rate ø(G), then the control target yaw rate Trø is constant at a certain value (see T3 in FIG. 7), and when after thatthe first and the second target yaw rates ø(θ, G) come to have the samesign, the one with the smaller absolute value of the first and thesecond target yaw rates ø(θ, G) (in FIG. 7, this is the second targetyaw rate ø(G)) is set as the control target yaw rate Tr ø (see T4 inFIG. 7).

[0150] Thus, the reason why the control target yaw rate Tr ø is kept ata constant value is to avoid that the control gain becomes too large,for example when the steering angle crosses the neutral point. Moreover,if the control target yaw rate Try ø stays set to the first target yawrate ø(θ) , then the control amount becomes large, and there is the riskthat the vehicle spins in the opposite direction. Thus, when the vehiclestarts to spin in the opposite direction, it becomes difficult torectify this opposite spin, so that when the values of the first andsecond target yaw rates ø(θ, G) have different signs, the target yawrate Tr ø is kept at a predetermined value.

[0151] It should be noted that if this predetermined value is set to theneutral point for example, then the vehicle can thereafter not cause aposture change in the yawing direction. For this reason, thepredetermined value is set to a value at a certain offset with respectto the neutral point.

Counter-Steering Converging Control

[0152] As mentioned above, in the case of an oversteering tendency, thedriver sometimes tries to counter-steer. Also in this case, suitablecontrol for rectifying the oversteering tendency is carried out, but dueto the braking control of the posture control, the posture change of thevehicle becomes larger than the corresponding steering wheel operation.Thus, sometimes an oversteering tendency in the opposite directionoccurs, caused by a delay of returning the steering wheel after thedriver has counter-steered, for example. As a result, there is the riskthat the posture of the vehicle in the yawing direction does notconverge.

[0153] In order to prevent such an oversteering tendency in the oppositedirection, a braking force is applied to the cornering inside frontwheel. FIG. 8 is a flowchart of the convergence control aftercounter-steering. First, in Step S31, it is judged whether oversteercontrol is still being carried out, or whether the step is within apredetermined time after such control. If the result of the judgment is“YES”, then the procedure advances to Step S32, and if it is “NO”, thenthe procedure returns.

[0154] Step S32 judges whether the driver is counter-steering or not.This judgment is carried out by judging whether the value of the actualyaw rate ø has become larger than the first target yaw rate ø(θ) basedon the steering angle or vice versa, or whether the steering angle speedhas reversed. If the result of this judgment is “YES”, then theprocedure advances to Step S33, whereas if it is “NO”, then theprocedure returns.

[0155] Step S33 judges whether the counter-steering amount is large.This can be done for example by judging whether the oversteeringtendency before the counter-steering is large (strong) or whether thesteering angle speed of the steering wheel is large when performing thecounter-steering. If the result of this judgment is “YES”, then theprocedure advances to Step S34, and if it is “NO”, then the procedurereturns.

[0156] Step S34 judges whether the steering angle speed has reversed ornot. This is done by judging whether, after counter-steering has beenperformed, the steering wheel is turned back into its original positionor not. If the result of this judgment is “YES”, then the procedureadvances to Step S35, and if it is “NO”, then the procedure returns.

[0157] In Step S35, it is judged whether the actual yaw rate ø isfollowing the change of the steering angle. That is to say, if theactual yaw rate ø is following the change of the steering angle, then itis likely that the yaw rate position is approaching convergence, so thatno braking force is applied to the cornering inside front wheel. If theactual yaw rate is following the change of the steering angle while thebraking force is applied, then it is also possible to stop theapplication of the braking force.

[0158] Then, if the result of the judgment at Step S35 is “NO”, then theprocedure advances to Step S36, and a braking force is applied to thecornering inside front wheel, whereas if it is “YES”, then the procedurereturns.

[0159] With this control, it can be avoided that the vehicle oversteersin the opposite direction after counter-steering has been performed.

Setting of the Threshold for the First Understeer Control

[0160] Referring to FIG. 9, the following explains how the thresholdTHUS of the first understeer control is set in Step S16 (see FIG. 2A).The threshold THUS is set by determining a basic threshold, which isthen amended.

[0161] First, at Step S41, a basic threshold is set. This basicthreshold should be set to a predetermined constant.

[0162] Then at Step S42, if the steering wheel is being returned, theintervention of control is suppressed (that is, intervention of controlis held back) by increasing the threshold in proportion with thesteering operation speed. This is because if the steering wheel isturned back although there is an understeering tendency, then it islikely that the driver understeers intentionally. If the driverundersteers intentionally, then it is preferable to suppress theintervention of control and to leave the operation up to the driver.Thus, by suppressing the intervention of control, the interference ofthe intervention of control into the operation of the driver can beavoided from the beginning.

[0163] Then, Step S43 suppresses the intervention of control byincreasing the threshold in proportion with the variation of the actualyaw rate ø(i.e. the change of the actual yaw rate). This is because ifthe actual yaw rate tends to increase, then the understeering tendencyis rectified. By contrast, when the control intervenes too early in thissituation, the change of the yaw rate becomes large, and as a result, anoversteering tendency may occur. Thus, in order to avoid an erroneousintervention of control in this case, the threshold is increased.

[0164] Step S44 suppresses the intervention of control by increasing thethreshold when the steering wheel is near the neutral position. This isbecause understeering usually occurs when the steering wheel has beenturned, and when the steering wheel is near the neutral position, it isnot necessary to suppress an understeering tendency. Thus, in order toavoid erroneous intervention in such a situation in which understeeringcan hardly occur, the threshold is increased to suppress theintervention of control.

[0165] Step S45 hastens the intervention of control by lowering thethreshold value accordingly when the lateral acceleration is small (in alow μ region). This is because during low μ, such as on an icy road,there is a greater chance of an understeering tendency, so that in thatcase, the posture control intervenes at an early stage.

[0166] Step S46 hastens the intervention of control by lowering thethreshold value accordingly when during cornering the second target yawrate ø(G) has decreased by at least a predetermined value. This aims athastening the intervention of control if the road surface μ decreasesabruptly and the vehicle slips sideways, for example when the roadsurface is partially frozen. In other words, when the road surface μchanges abruptly, the driver cannot operate the steering wheel, or along time is necessary until the driver operates the steering wheel. Inthis situation, if the posture control is performed using for exampleonly the first target yaw rate ø(θ), it becomes impossible to start theposture control, because the first target yaw rate ø(θ) does notfluctuate. By contrast, in this embodiment, the posture control iscarried out using the second target yaw rate ø(G) which is based on thelateral acceleration, so that it becomes possible to perform precisecontrol early on, even under such fluctuations of the road surface μ.

[0167] In this manner, the threshold THUS of the first understeercontrol is set.

Setting of the Threshold for the First Oversteer Control

[0168] Referring to FIG. 10, the following explains how the thresholdTHOS of the first oversteer control (first intervention threshold) isset in Step S16 (see FIG. 2A). Also this first intervention thresholdTHOS is set by determining a basic threshold, which is then amended.

[0169] First, at Step S51, a basic threshold is set. As shown in FIG.11, the basic threshold is set to a larger value the smaller the vehiclespeed V is. And when the speed is extremely low, the basic threshold isset to an even higher value.

[0170] Then, in Step S52, as shown in FIG. 12, the threshold is amendedto a higher value the higher the lateral acceleration is, and theamendment is larger the larger the vehicle speed is. This is becausewhen for example the lateral acceleration is low, that is to say inregions with low μ, an oversteering tendency occurs more easily, so thatintervention of control is hastened by setting a low threshold.Conversely, if the lateral acceleration is high (in high μ regions) andthe driving speed is high, then the posture of the vehicle changesquickly, so that when the threshold is low, erroneous intervention ofposture control occurs easily. Furthermore, a driver who can maneuverthe vehicle at high speeds on high μ regions is likely to be able todeal with slight posture changes of the vehicle. Therefore, theinterference of posture control into the driver's operation should beprevented, and a high threshold is set in high lateral accelerationregions and in high sped regions.

[0171] At Step S53, intervention of control is suppressed by increasingthe threshold in inverse proportion to the steering angle. For example,even when the steering angle is small, it may occur that the orientationof the vehicle and the orientation of the steering wheel are opposite,in particular due to such disturbances as an icy road. In such a case,the vehicle automatically assumes a stable driving orientation withoutperforming posture control, so that the intervention of control issuppressed.

[0172] At Step S54, the intervention of control is suppressed byincreasing the threshold, the increase being larger the slower thesteering wheel is returned. This is because if the driver returns thesteering wheel slowly, then it is likely that the driver himself canadequately rectify an oversteering tendency without the intervention ofcontrol. Therefore, the threshold suppressing the intervention ofcontrol is increased.

[0173] Then, at Step S55, the intervention of control is suppressed byincreasing the threshold when the yaw rate overshoots. “Overshooting ofthe yaw rate” means that, as shown in FIG. 13, when the steering wheelis returned from a turned orientation to the neutral point, the actualyaw rate ø overshoots even though the vehicle is not in an unstablestate. In such a case, it is judged that there is an oversteeringtendency, so that the threshold is increased to suppress theintervention of control.

[0174] At Step S56, if the variations of the yaw rate are large, theintervention of control is suppressed by increasing the threshold. Thepurpose of this is to prevent erroneous intervention of control.

[0175] At Step S57, the threshold is lowered in case thatcounter-steering or tuck-in in a front-wheel driven vehicle is judged,in which the front wheels are the driven wheels, is performed. Here,tuck-in is judged to be the case when for example the followingconditions are satisfied: The steering angle is at a constant turnedorientation, the car is in the lower second or third gear, and theaccelerator pedal has returned and the throttle opening is small.Counter-steering is judged by the steering wheel angle.

[0176] Then in Step S58, when the basic threshold has been increased bythe above-described steps, there is the risk that the threshold has beenset to a value that is too high, so that an upper limit is set. Thus,the first intervention threshold THOS serving as the threshold for thefirst oversteer control is set.

Termination Judgment of the First Oversteer Control

[0177] Next, referring to the flowchart shown in FIG. 14, thetermination judgment of the first oversteer control (see Step S12 inFIG. 2B) is explained. The purpose of this control is to avoidinterference of posture control into the driver's operation, whilemaking sure that the posture control is terminated in a position inwhich the posture of the vehicle is stable.

[0178] First, Step S61 judges whether the steering wheel has beenstabilized such that the vehicle advances straight forward, or in otherwords, whether the steering angle has been stabilized substantially atthe neutral position. If the result of the judgment is “NO”, then theprocedure advances to Step S62.

[0179] Step S62 judges whether the steering wheel has been turned. Ifthe judgment is “NO”, then the procedure advances to Step S63.

[0180] Step S63 judges whether the deviation between the second targetyaw rate ø(G) and the actual yaw rate ø has been stabilized at below apredetermined value. That is to say, it is judged whether both valuesare sufficiently low, and almost matching. If the judgment is “NO”, thenthe procedure advances to Step S65.

[0181] Then, if the judgment at any of Steps S61 to S63 is “YES”, thenthe procedure advances to Step S64, the control is terminated and theprocedure returns. With the judgment of Step S61, it is likely that thedriver is operating the steering wheel calmly, so that it is notnecessary to perform posture control. On the contrary, if posturecontrol is performed in this situation, then there is the risk that theposture control interferes with the driver's operation. At the judgmentat Step S62, judging the fact that the driver turns the steering wheelin the direction promoting the oversteering tendency, it is likely thatthe driver intentionally corners provoking an oversteering tendency, orthat the driver intentionally lets the vehicle spin, for example toavoid an accident. In this case, interference of the posture controlwith the driver's operation is prevented by quickly terminating theposture control. Moreover, at the judgment at Step S63, judging the factthat the vehicle is in a stable state in which the second target yawrate ø (G) substantially matches the actual yaw rate ø, it can beconcluded that the posture of the vehicle has been stabilized.Consequently, there is no necessity to perform posture control, so thatthe control is terminated.

[0182] Then, Step S65 judges whether the estimated brake pressure, whichis estimated from the braking amount at the posture control, issubstantially the same as the pressure in the master cylinder. That isto say, it is judged whether it is likely that the posture control canbe terminated substantially without controlling the braking force. Ifthe judgment is “YES”, then the procedure advances to Step S66, whereasif it is “NO”, then the procedure advances to Step S69.

[0183] Step S66 judges whether the slip angle β is small. That is tosay, it judges whether lateral slipping is occurring or not. If thejudgment is “YES”, then the procedure advances to Step S67, and if it is“NO”, then the procedure returns without terminating the control.

[0184] Step S67 judges whether the second target yaw rate ø (G), thefirst target yaw rate ø(θ) and the actual yaw rate ø are all below apredetermined value. That is to say, it is judged whether these threevalues are smaller than a predetermined value and are nearing eachother. This judgment judges whether the vehicle is travelingsubstantially straight forward, the steering wheel is not beingoperated, and there is no need to perform posture control. That is tosay, since the conditions of Step S63 are sometimes difficult tosatisfy, this judgment terminates the posture control under conditionsthat are looser than the conditions of Step S63. Then, if the judgmentis “YES”, the procedure advances to Step S68, which judges whether apredetermined period of time T1 has elapsed after the afore-mentionedconditions have been fulfilled. That is to say, it is conceivable thatthe conditions are accidentally fulfilled, so that it is judged whethera predetermined period of time has elapsed. If the judgment is “YES”,then the procedure advances to Step S612, the posture control isterminated, and the procedure returns.

[0185] Step S69 judges whether the slip angle β is small. If thejudgment is “YES”, then the procedure advances to Step S610.

[0186] Step S610 judges whether two of the second target yaw rate ø(G),the first target yaw rate ø(θ) and the actual yaw rate ø are below apredetermined value, and the remaining value is not very different froma predetermined value. This is a condition that is looser than thecondition of Step S67. If the judgment is “YES”, then the procedureadvances to Step S611, which judges whether a predetermined period oftime T2 has elapsed after the conditions of Step S610 have beenfulfilled. The predetermined period of time T2 is longer than thepredetermined period of time T1 in Step S68, because the condition islooser than the condition of Step S67. Then, if the judgment is “YES”,then the control is terminated, and the procedure returns.

[0187] On the other hand, if the judgment at Steps S67, Step S68, StepS610, and Step S611 is in all cases “NO”, then the control is continuedand the procedure returns.

[0188] By continuing the control until the vehicle is in a stabledriving state, it can be prevented that cases occur, in which thetermination of the control is judged based only on the deviation betweenthe control target yaw rate Tr ø and the actual yaw rate ø, leading to atoo early termination of the posture control.

[0189] Moreover, such a termination judgment of the posture control isalso useful when continuing posture control is necessary after posturecontrol has been performed once, for example avoiding an obstacle. Byrepeating termination and begin of control in a short period of time,the risk of posture changes brought about by the termination of theposture control, as well as unstabilities of the driving operation canbe prevented by continuing the control until the vehicle is in a stabledriving state.

[0190] On the other hand, in situations in which the driver does notneed the control, an interfering of the posture control with theoperation of the driver can be avoided by terminating the posturecontrol early.

Control of the Brake Fluid Pressure

[0191] Referring to the flowchart shown in FIG. 15, the followingexplains the braking fluid pressure (hydraulic) control for the firstoversteer control and the first understeer control. In this brakingfluid control, the pressure is not feedback controlled, but controlledin two phases. In a first phase, the braking fluid is pressurized with apredetermined pressurizing (pressure-increasing) speed, and in a secondphase (pressure adjustment state), the braking fluid is adjusted whenbraking force is applied by pressurizing the braking fluid to change theposture of the vehicle.

[0192] First, Step S71 judges whether behavior control (that is, firstoversteer control or first understeer control) has been started or not.Then, Step S72 judges whether oversteer control is carried out. If thejudgment is “YES” (in case of oversteer), then the procedure advances toStep S73, and if it is “NO”(in case of understeer), then the procedureadvances to Step S74.

[0193] At Step S73, the brake pressure is pressurized with apressurizing speed at the mechanical limit (hydraulic pressure MAX).That is to say, the pressurizing pump 32 is operated at the mechanicallimit. The pressurizing is carried out after completely opening the ASWsolenoid valve 36 as well as the front and rear solenoid valves 33 and34 arranged in the supply path to the wheels to which a braking force isapplied.

[0194] Step S77 judges whether the slip ratio has at least apredetermined value or not. Here, the slip ratio can be calculated basedon the estimated vehicle speed and the wheel speed obtained from thedetection signal of the wheel speed sensors 11. This judgment isperformed with the purpose of preventing an excessive braking fluidpressure. That is to say, when the slip ratio has at least apredetermined value, then the braking fluid pressure becomes too high ifthe braking fluid is further pressurized. If the judgment is “NO”, thenthe procedure advances to Step S78.

[0195] Step S78 judges whether the peak of the acceleration of thechange of the slip angle β has already passed. If the judgment is “YES”,then the procedure advances to Step S79, and if it is “NO”, then theprocedure advances to Step S710.

[0196] Step 579 judges whether either one of the change rate (changespeed) of the yaw rate deviation Δø(θ, G) and the change acceleration ofthe yaw rate deviation Δø(θ, G) is decreasing, that is whether the yawrate deviation is changing in a convergence direction.

[0197] Step S710 judges whether either one of the change rate of theslip angle β and the change acceleration of the slip angle β isdecreasing even though the peak of the slip angle has not passed, thatis, whether the slip angle is changing in a convergence direction.

[0198] Steps S78 to S710 judge whether the posture of the vehicle haschanged due to the application of braking force by pressurizing thebraking fluid, that is, whether the effect of posture control has beenattained or not.

[0199] Then, if the judgment at any of Step S77, Step S79 and Step S710is “YES”, the procedure advances to Step S711, which judges whether apredetermined period of time T4 has elapsed after starting to pressurizethe braking fluid. This predetermined period of time T4 should be set inconsideration of the starting threshold of the posture control and thecharacteristics of the braking fluid pressurizing control system, suchas the pressurizing pump 32. That is to say, the predetermined period oftime T4 should be set to the time that, judging by the characteristicsof the braking fluid pressurizing system, is minimally necessary toincrease the pressure to the necessary braking fluid pressure. Then, ifthe judgment is “YES”, the procedure advances to Step S712, and, as thesecond phase, goes to a state of adjusting the pressure, that is, astated in which the pressure of the braking fluid is kept or increasedor reduced in accordance with the current state. If the judgment is“NO”, then the procedure returns, and the pressurizing is continued.

[0200] On the other hand, if the procedure has advanced to Step S74 incase of understeer control, then first, at Step S74, the braking fluidis pressurized with a pressurizing speed at the mechanical limit. Then,Step S75 judges whether a predetermined period of time T3 has elapsedafter starting to pressurize the braking fluid. If the judgment is“YES”, then the procedure advances to Step S76, and if it is “NO”, thenthe pressurizing is continued at a pressurizing speed at the mechanicallimit until the predetermined period of time T3 has elapsed afterstarting to pressurize the braking fluid. On the other hand, at StepS76, the braking fluid is pressurized for example at a speed of(pressurizing speed at mechanical limit×0.8).

[0201] This control of the braking fluid pressure is to avoid rocking ofthe wheels, since the tires do not have gripping power during anundersteering tendency. That is to say, by first pressurizing thebraking fluid with a pressurizing speed at the mechanical limit, thelagging of the braking fluid pressure with respect to the posturecontrol, like break pads adhering on the disk rotor, is rectified. Then,the pressurizing speed is somewhat reduced and the pressurizing iscontinued. Thus, it can be avoided that an excessive braking fluidpressure is applied so that the wheels rocks.

[0202] Step S713 judges whether the slip ratio has at least apredetermined value. If the judgment is “NO”, then the procedureadvances to Step S714, which judges whether actual yaw rate ø isfollowing the turning operation of the steering wheel. If the judgmentis “NO”, then the procedure returns and the pressurizing is continued,because the effect of the posture control does not manifest itself.

[0203] On the other hand, if at either of Step S713 and Step S714 thejudgment is “YES”, then the procedure advances to Step S715, whichjudges whether a predetermined period of time T5 has elapsed afterstarting to pressurize the braking fluid. If the judgment is “YES”, thenthe procedure advances to Step S716, and goes to a pressure adjustmentstate. If the judgment is “NO”, then the pressurizing should becontinued and the procedure returns.

[0204] By controlling the braking fluid pressure in this manner withoutperforming a feedback control, it is easy to achieve a braking fluidpressure control system.

[0205] Moreover, by first pressurizing the braking fluid with apressurizing speed at the mechanical limit or at a pressurizing speedlower than the mechanical limit (first phase), the braking power isapplied early, so that a quick posture control can be achieved. Inaddition, when the vehicle posture goes into convergence direction,switching to pressure adjustment control of the braking fluid pressure(second phase) makes it possible to achieve precise posture controlwithout the control amount becoming excessive.

[0206] In particular, in the case that the intervention of the firstoversteer control and the first understeer control is delayed as much aspossible as in this embodiment, the driver will rarely feel awkward whenthe braking fluid pressure is controlled in this manner. Furthermore,this control of the braking fluid pressure is extremely useful in thatquick posture control becomes possible.

Control of the Warning Apparatus

[0207] Referring to the flowchart of FIG. 16, the following explains thecontrol of the warning apparatus 38. The start of the operation of thewarning apparatus 38 is delayed after the start of the posture control(behavior control), and also the end of the operation of the warningapparatus 38 is delayed after the end of the behavior control.

[0208] First, Step S81 judges whether a flag F is 1 or not. As explainedbelow, this flag F is 1 when behavior control of the vehicle is beingperformed. Then, if the judgment is “YES”, the procedure advances toStep S87, and if it is “NO”, then the control of the operation start ofthe warning apparatus should be performed and the procedure advances toStep S82.

[0209] Step S82 judges whether behavior control is currently beingperformed. If the judgment is “YES”, then the procedure advances to StepS83, and if it is “NO”, then the procedure returns.

[0210] Step S83 judges whether the estimated braking fluid pressure hasat least a predetermined value. If the judgment is “YES”, then theprocedure advances to Step S84, and if it is “NO”, then the procedureadvances to Step S85.

[0211] Step S85 judges whether a predetermined period of time haselapsed after the start of the behavior control. If the judgment is“YES”, then the procedure advances to Step S84, and if it is “NO”, thenthe procedure returns.

[0212] At Step S84, the flag F is set to 1 and the procedure returns toStep S86, the warning apparatus is activated (warning ON), and theprocedure returns.

[0213] Thus, the begin of the operation of the warning apparatus isdelayed after the begin of the behavior control, until for example theestimated brake pressure has reached at least a predetermined value oruntil the behavior control has been carried out for at least apredetermined time. This prevents that the warning apparatus is operatedeven though the driver has not noticed the intervention of behaviorcontrol, so that an awkward feeling of the driver as well as operationalmistakes caused by this awkward feeling can be prevented.

[0214] The above-described Steps S82 to S86 constitute the controlrelated to the operation start of the warning apparatus 38, whereas thecontrol performed if the judgment at Step S81 is “YES” is related to theending of the operation of the warning apparatus 38.

[0215] First, Step S87 judges whether the vehicle is traveling straightforward and is in a stable state. If this judgment is “NO”, then theprocedure advances to Step S88.

[0216] Step S88 judges whether a predetermined period of time haselapsed after the behavior control has been terminated. If the judgmentis “NO”, then the procedure advances to Step S89.

[0217] Step S89 judges whether the braking fluid pressure (brakepressure) is substantially the same as the pressure in the mastercylinder, that is to say, whether the braking fluid pressure is atatmospheric pressure when for example the driver is not depressing thebrake pedal, or conversely whether the braking fluid pressure is at thepressure of the master cylinder, in correspondence to the depressingamount of the brake pedal, when the driver is depressing the brakepedal. If the judgment is “NO”, then the procedure returns.

[0218] If the judgment at any of Step S83, Step S88 and Step S89 is“YES”, then the procedure advances to Step S810, the flag F is set to 0,the operation of the warning apparatus 38 is terminated at Step S811,and the procedure returns.

[0219] Thus, by terminating the operation of the warning apparatus 38after a predetermined period of time has elapsed after the terminationof the behavior control, the warning is performed continuously withoutrepeatedly terminating and starting the warning when a behavior controlis carried out intermittently, for example to avoid an obstacle. Thus,it is possible to prevent an awkward feeling of the driver.

[0220] Furthermore, by continuing the operation of the warning apparatus38 after terminating the behavior control until the driving environmentof the vehicle changes, for example when the vehicle stabilizes in astate of traveling straight forward, or when the brake pressure fluidsubstantially matches the pressure in the master cylinder, it can beprevented that the warning is repeatedly terminated and started. As aresult, a suitable warning can be achieved without the driver feelingawkward.

Other Embodiments

[0221] It should be noted that the present invention is not limited tothe above-described embodiment, and a variety of other embodiments areincluded in the present invention. For example, in the above embodiment,in the setting of the threshold THUS of the first understeer control(see FIG. 9) if the second target yaw rate ø(G) decreases below apredetermined value during cornering, the threshold is decreased (seeFIG. 9, Step S46). However, it is also possible to forcibly intervenewith the first understeer control in a case corresponding to the aboveconditions without amending the threshold THUS, and to begin thecontrol.

[0222] Furthermore, in the above-described embodiment, in the setting ofthe threshold THOS for the first oversteer control (see FIG. 10), thethreshold is set low in the case of tuck-in (see FIG. 10, Step S57), butdifferent from that, it is also possible to forcibly intervene with thefirst oversteer control in the case of tuck-in, and start the control.That is to say, in Step S19 in FIG. 2, it is also possible to judgewhether the yaw rate deviation Δø(θ,G) has exceeded the threshold orwhether there is tuck-in.

[0223] Moreover, in the above-described embodiment, in the case ofcounter-steering, the first intervention threshold THOS is lowered (seeStep S57 in FIG. 10), but different from that, it is also possible toforcibly intervene with the first oversteer control in the case ofcounter-steering, as in the case of tuck-in, and to start the control.

[0224] In addition, if the first target yaw rate ø(θ) has become smallerthan the second target yaw rate ø(G), for example when the driver hascounter-steered during an oversteering tendency (see FIG. 7), then, inthe above embodiment, the control target yaw rate Tr ø is changed fromthe second to the first target yaw rate at the time when the firsttarget yaw rate ø(θ) has become smaller than the second target yaw rateø(G). However, different from that, it is also possible to perform thecontrol for example as follows.

[0225] When the control target yaw rate Tr ø has been changed from thesecond target yaw rate ø(G) to the first target yaw rate ø(θ), there isalso the risk that the brake pressure changes abruptly. Therefore, whenit is predicted, based on the fact that the steering angle has reversed,that the absolute value of the first target yaw rate ø(θ) becomessmaller than the second target yaw rate ø(G), then it is also possibleto relax the control amount, so that the control target yaw rate Tr ødoes not change abruptly. That is to say, the control operation isrelaxed when the control target yaw rate Tr ø has been switched form thesecond target yaw rate ø(G) to the first target yaw rate ø(θ).

[0226] An example of a means for relaxing the control operation ispreviously setting an upper limit of the brake pressure and making surethat the brake pressure does not exceed this upper limit even when thecontrol target yaw rate Tr ø has been changed from the second target yawrate ø(G) to the first target yaw rate ø(θ). Another example of a meansfor relaxing the control operation is setting the control target yawrate Tr ø by adding, as an amendment of the control target yaw rate Trø, the derivative of the first target yaw rate ø(θ) to the second targetyaw rate ø(G) when it is predicted that the first target yaw rate ø(θ)becomes smaller than the second target yaw rate ø(G) When this is done,then the control operation is relaxed when switching the control targetyaw rate Tr ø, and the shock due to the switching can be reduced.

[0227] Furthermore, in the above-described embodiment, the controltarget yaw rate Tr ø is set to the first or second target yaw rate ø(θ,G) that has the smaller absolute value. But different from that, it isalso possible that when the yaw rate variations are extremely high, suchas when driving on a poor road, the control target yaw rate Tr ø is setto the first target yaw rate ø(θ) even though the absolute value of thesecond target yaw rate ø(G) is smaller than the absolute value of thefirst target yaw rate ø(θ). This is because if the yaw rate variationsare extremely high, the variations of the lateral acceleration are veryhigh, and there is the risk that the second target yaw rate ø(G) is notsuitable as the control target yaw rate Tr ø. Therefore, if the yaw ratevariations are extremely high, it is also possible to take the firsttarget yaw rate ø(θ) based on the steering angle, which takes on astable value, as the control target yaw rate Tr ø.

[0228] Moreover, if the yaw rate variations are extremely high, it isalso possible to use the following equation instead of theafore-mentioned Equation (3) as the equation for amending the controltarget yaw rate Tr ø.

Trø=(1−k2)×ø(G)+k 2×ø(θ)  (4)

[0229] That is to say, the control target yaw rate Tr ø is set to thesecond target yaw rate ø(G) plus an amendment value that is proportionalto the difference between the first target yaw rate ø(θ) and the secondtarget yaw rate ø(G). When k2 is large, the amendment ratio of the firsttarget yaw rate ø(θ) becomes large, and it becomes possible to perform asuitable posture control even when the yaw rate variations are extremelylarge.

[0230] Furthermore, in the above-described embodiment, a condition forstarting the operation of the warning apparatus 38 is that the estimatedbraking fluid pressure has at least a predetermined value (Step S83 inFIG. 16). In addition to this condition, it is also possible to let thewarning apparatus 38 operate when for example the decrease of the engineoutput has at least a predetermined value.

[0231] Furthermore, the above-mentioned embodiment includes, asoversteer controls, a second and a third oversteer control in additionto the first oversteer control. However, the present invention is notlimited to this, and it is also possible to provide only the firstoversteer control and either one of the third and the second oversteercontrol.

[0232] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Theembodiments disclosed in this application are to be considered in allrespects as illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims rather than by the foregoingdescription, all changes that come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

What is claimed is:
 1. A vehicle posture control apparatus provided witha control means for controlling posture of the vehicle in yawingdirection by independently controlling brakes of the vehicle's wheels,wherein the control means carries out intervention of a first oversteercontrol suppressing an oversteering tendency when the oversteeringtendency of the vehicle is stronger than a first preset reference value;and wherein the control means carries out intervention of a secondoversteer control, in which the control amount is lower than in thefirst oversteer control, when the oversteering tendency of the vehicleis not stronger than the first preset reference value, but stronger thana second preset reference value.
 2. The vehicle posture controlapparatus according to claim 1, wherein the second oversteer controlsupplies, by open control, a brake pressure whose upper limit is apredetermined brake pressure that is lower than the maximum brakepressure that can be supplied in the first oversteer control.
 3. Thevehicle posture control apparatus according to claim 1, wherein thesecond oversteer control supplies brake pressure in accordance with adeviation between a target yaw rate that has been set and the actual yawrate.
 4. The vehicle posture control apparatus according to claim 1,wherein the control means prohibits intervention of the second oversteercontrol when the vehicle has an understeering tendency.
 5. A vehicleposture control apparatus provided with a control means for controllingposture of the vehicle in yawing direction by independently controllingbrakes of the vehicle's wheels, wherein the control means carries outintervention of a first oversteer control suppressing an oversteeringtendency when the oversteering tendency of the vehicle is stronger thana first preset reference value; wherein the control means carries outintervention of a second oversteer control, in which the control amountis lower than in the first oversteer control, when the oversteeringtendency of the vehicle is not stronger than the first preset referencevalue, but stronger than a second preset reference value; and whereinthe control means carries out intervention of a third oversteer control,in which the control amount is lower than in the first oversteercontrol, when the oversteering tendency of the vehicle is not strongerthan the second preset reference value, but stronger than a third presetreference value.
 6. The vehicle posture control apparatus according toclaim 5, wherein a supply ratio of brake pressure during the thirdoversteer control is set to be lower than a supply ratio of brakepressure during the second oversteer control.
 7. The vehicle posturecontrol apparatus according to claim 5, wherein the second oversteercontrol supplies, by open control, a brake pressure whose upper limit isa predetermined brake pressure that is lower than the maximum brakepressure that can be supplied in the first oversteer control.
 8. Thevehicle posture control apparatus according to claim 5, wherein thethird oversteer control supplies brake pressure in accordance with adeviation between a target yaw rate that has been set and the actual yawrate.
 9. The vehicle posture control apparatus according to claim 5,wherein upper limits of the brake pressure in the second and thirdoversteer control is set to 10 to 25 bar.
 10. The vehicle posturecontrol apparatus according to claim 5, wherein the control meansprohibits intervention of the second oversteer control when the vehiclehas an understeering tendency.
 11. The vehicle posture control apparatusaccording to claim 5, wherein the control means prohibits interventionof the third oversteer control when the vehicle has an understeeringtendency.